The invention relates to an induction-heated godet with a stationary, circularly cylindrical heater and a circularly cylindrical godet jacket coaxially surrounding the heater and rotatable about the heater around a common longitudinal axis, where the heater has a core, preferably consisting of lamellar pieces of sheet metal, with at least three recesses, arranged one after the other in an axial direction, the recesses being open toward the godet jacket, and with a primary winding in each recess, and where the godet jacket has a magnetic return jacket for magnetic flux which reaches into areas between each pair of recesses to the inside periphery of godet jacket, and further, the godet jacket has a secondary winding coaxially disposed within the magnetic return jacket for each primary winding, especially at least one short-circuiting ring made from electrically conductive but magnetically inactive material.
Induction-heated godets serve as draft rolling units, stretch rollers or the like, and are used, for example, in chemical fiber production, sheet production and paper production (cf., for example, EP-A 0 349 829). For godets of the type being discussed, it is essential that the outside surface of the cylindrical godet jacket be at a reactively high temperature to make possible stretching and shrinking of the materials coming in contact with the godet jacket.
Heating of the circularly cylindrical godet jacket, which in operation rotates at high speed around its longitudinal axis, takes place by a heater placed stationary inside the godet jacket. The heater can cause the heating of the godet jacket by heat radiation, heat convection or by induction processes. An inductive heater has proved to be particularly effective, and this invention relates to such an inductive heater.
With an induction-heated godet, i.e., a godet with an inductive heater, a magnetic flux, which is produced by the stationary primary windings on the core of the heater, is enclosed in the godet jacket in the magnetic return jacket made of ferromagnetic material, mostly of iron. In the short-circuiting ring or short-circuiting rings, voltages are induced, which lead to corresponding currents, by which the short-circuiting rings and with it the godet jacket are heated as a whole.
Due to different rates of heat dissipation from the godet jacket, and also as a result of differences inside the godet jacket, different local outputs, etc., temperature differences result on the outside of the godet jacket over its width. These temperature differences must be as small as possible if a good temperature profile is to be kept. For this purpose, with induction-heated godets, steam./liquid systems have been used with only one primary winding; however, these systems are limited to a use range of temperatures up to about 500.degree. K., they age quickly and are not optimal from a wear viewpoint. Circulating systems working with active pumps cannot be used in operationally rapidly rotating godet jackets.
To achieve a good temperature profile even at temperatures of 600.degree. to 750.degree. K., which are necessary for the production of modern industrial synthetic fibers, the primary winding in the heater has been divided into several individual primary windings, mostly two or three primary windings. In this way, a considerable improvement of the temperature profile is obtained, since the working width of the godet jacket is divided into individual sections, which in each case can be adjusted by themselves. The working temperature is not limited by the steam/liquid system, and considerably higher peripheral speeds of the godet are possible.
Also, in the induction-heated godet explained above with several primary windings placed successively in the axial direction, from which the invention starts (DE-A 3 527 271), a temperature profile with a temperature band width of 3.degree. to 4.degree. K. for some uses is not yet optimal.